Freeze drying is a process that removes water from a product in the form of ice. In a freeze drying process, the product is frozen and, under vacuum, the ice sublimes and the resulting vapor flows towards a condenser. Ice subsequently condensed on the condenser is removed in a later stage. Freeze drying is particularly useful in the pharmaceutical industry, as the integrity of the product is preserved during the freeze drying process and product stability can be guaranteed over relatively long periods of time.
Freeze dryers have a vacuum chamber for receiving a plurality of containers or vials containing the product to be freeze dried. The chamber usually includes a number of shelves, which can be raised and lowered within the chamber. To load the shelves, the shelves are initially moved to the lower portion of the chamber, with the uppermost shelf in a loading position. After that shelf has been loaded, the mechanism automatically raises the shelves to permit the next shelf to be moved to the loading position. The moving sequence continues until the chamber loading has been completed. To unload the chamber, the loading sequence is reversed, with the lowermost shelf being unloaded first.
Access to the chamber for automated loading and removal of product is through a rectangular opening, or slot, formed in a wall or in a main door of the chamber. A moveable slot door closes the slot. To permit vials of product to be inserted into the chamber, the slot door is vertically raised relative to the slot by moving the slot door along guide tracks.
Various techniques have been used in loading and unloading vials in the chamber. In one arrangement, a mechanism pushes vials onto a shelf in the chamber from a conveyor belt running parallel with the slot. The vials may be loaded from the conveyor belt row by row onto a shelf. Another loading technique utilizes a wheeled transfer cart that moves on rails. The transfer cart may carry an entire shelf of vials at a time, which may be pushed onto the shelf in the chamber in a single loading operation. After a shelf is completely loaded, the shelf is raised and another is moved into position for loading. Once loading has been completed, the slot door is lowered to close the slot and enable the chamber to be evacuated for subsequent freeze drying of the contents of the vials.
Several actuation mechanisms have been used to move the slot door of a freeze drying chamber between the open and closed positions. In one arrangement, one or more lead screws or ball screws are used to raise and lower the slot door in its guide tracks. The screws are arranged on either side of the slot door. In another arrangement, one or more pneumatic or hydraulic piston actuators are used to raise and lower the slot door.
The slot door and its actuation mechanism reside in a class 100, or class A, sanitary clean room environment. In some applications, one or more of the loading and door actuation components, together with the chamber opening, may reside in an isolator that contains a sterile environment, the isolator allowing the room it is located in to have less stringent cleanliness requirements, such as class C.
The chamber, slot door and actuation mechanism must periodically be sanitized or sterilized to maintain the environment. Both screw-type and piston-type actuators include internal mechanisms that are not easily sanitized using direct chemical or steam sanitizing techniques. For example, ball screws contain ball bearings, ball recirculation tracks and internal ball grooves that cannot easily be sanitized without disassembly. Similarly, the threads of a lead screw and the cylinder rod, cylinder wall and piston of a hydraulic or pneumatic actuator are not easily accessed for sanitization.
To maintain the aseptic nature of the clean environment, those linear actuation mechanisms may be placed outside the clean environment. In that case, a link having a linear motion must traverse the barrier between the clean processing environment and the external, ambient environment. The barrier must therefore incorporate a moveable seal such as a flexible bellows or a sliding cover to permit movement while enclosing the sanitary environment. Both the bellows and the sliding covers have shortcomings. Bellows are difficult to sanitize manually because they present an internal geometry with numerous corrugations. That especially presents challenges in applications where sanitization is done by spraying. Sliding covers also present problems inasmuch as they move into and out of the sanitary environment and cannot be hermetically sealed.
There therefore remains a need for a slot door actuation method and apparatus with improved characteristics for maintaining a clean environment.